Abstract

The chemical shift of acetone was measured in aqueous solution at high temperatures up to 400 °C and water densities of 0.10–0.60 g/cm3 for the study of hydration structure in the supercritical conditions. The average number of hydrogen bonds (HBs) between an acetone and solvent waters and the energy change upon the HB formation were evaluated from the δ and its temperature dependence, respectively. At 400 °C, is an increasing function of the water density, the increase being slower at higher water densities. The acetone–water HB formation is exothermic in supercritical water with larger negative at lower water densities (−3.3 kcal/mol at 0.10 g/cm3 and −0.3 kcal/mol at 0.60 g/cm3), in contrast to the positive in ambient water (+0.078 kcal/mol at 4 °C). The corresponding Monte Carlo simulations were performed to calculate the radial and orientational distribution functions of waters around the acetone molecule. The density dependence of calculated at 400 °C is in a qualitative agreement with the experimental results. In the supercritical conditions, the HB angle in a neighboring acetone–water pair is weakly influenced by the water density, because of the absence of collective HB structure. This is in sharp contrast to the hydration structure in ambient water, where the acetone–water HB formation is orientationally disturbed by the tetrahedral HB network formation among the surrounding waters.